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studied are carefully defined so that it is evident how an independent investigator could identify the
same outcome in a different study population. Outcomes are measured either by objective means
or by self-report. For self-reported outcomes, however, there are explicit criteria for how the data
were collected and evidence that the collection method would permit another investigator to repeat
the study in another population.
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The exposure measures are well defined. Self-report of exposure is acceptable so long as the method
of collecting self-reports was well specified and there was evidence that the self-reports were reliable
reflections of exposures. Job titles as surrogates for exposure were acceptable when the exposure of
interest was inherent in the job (e.g., vibration exposure for those operating pneumatic chipping
hammers).
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The article was published in English.
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The article was peer reviewed.
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The study was done within the last 20 yr (preferably).
No specific limitations were placed on study designs acceptable for consideration. The advantages of
prospective studies, however, were recognized. For example, there were sufficient prospective studies of
low back pain to examine these separately among the studies of physical factors and exclusively among
the studies of psychosocial factors.
2.1.2 Literature Search Methods
The literature reviews were conducted using computer-based bibliographic databases, with MEDLINE
(National Library of Medicine, U.S.A) a component of all searches. Additional databases included:
NIOSHTIC (National Institute for Occupational Safety and Health, U.S.A), HSELINE (Health and
Safety Executive, U.K), CISDOC (International Labour Organization, Switzerland), Ergoweb (Internet
site of the University of Utah), Psychinfo, Oshrom, Ergonomics Abstracts, and ArbLine (National Insti-
tute for Working Life, Sweden).
The bibliographies of articles (particularly review articles) and the NIOSH comprehensive review
(Bernard, 1997b) were examined to identify additional relevant articles.
Using these sources, a candidate list of articles were established and then systematically screened to
determine which ones met the strict criteria, described above, for inclusion in the review. Each
process reduced the list substantially. For physical work factors studied in association with back dis-
orders, 255 studies were initially identified as relevant and 41 met the selection criteria and were
reviewed. For psychophysical factors and back disorders, the search resulted in 975 references, which were
then reduced to 21 work-related risk factor studied and 29 individual risk factor studies. For work-related
physical factors and upper extremity disorders, the initial list of 265 references was reduced to 13 that
provided direct and 29 that provided indirect measures of exposure. For psychophysical factors and
upper extremity disorders, the initial 120 references were reduced to 28.
2.1.3 Analysis of Study Results
2.1.3.1 Definition of Measure: Relative Risk
In epidemiology, the relative risk is a measure of the strength of an association, here meaning the relation-
ship between the frequency of an exposure and the occurrence of an outcome (e.g., amount of vibration
and incidence of back pain). Because human populations typically have a variety of exposures occurring
in near proximity, relative risk is typically measured as the incidence of disease in the exposed (e.g., heli-
copter pilots who experience vibration) and the incidence of disease in the unexposed (similar people,
like ground crews, who are considered to share nearly the same other exposures as the exposed, such
as recreational activities, diet, and living conditions). The ratio of incidence provides a measure of associ-
ation, and the higher this ratio of incidence (the relative risk), the stronger the association, the more con-
fidence we can place in a conclusion that the association is meaningful.
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